Magneto-rheological fluid damper

ABSTRACT

A damper includes a cylinder that seals a magneto-rheological fluid, a piston slidably disposed in the cylinder, and a piston rod coupled to the piston. The piston includes a piston core, a flux ring, a plate, and a fixing nut. The piston core is mounted on the piston rod. The piston core has an outer periphery on which a coil is disposed. The flux ring forms a flow passage for the magneto-rheological fluid with the piston core. The plate is disposed on an outer periphery of the piston rod, has an outer peripheral surface housed in a one-end of the flux ring, and is bonded on the flux ring by brazing. The fixing nut sandwiches the plate with the piston core.

TECHNICAL FIELD

The present invention relates to a magneto-rheological fluid damper thatuses a magneto-rheological fluid whose apparent viscosity varies due toan action of a magnetic field.

BACKGROUND ART

As a damper mounted on a vehicle such as an automobile, there is adamper where a magnetic field is activated on a flow passage throughwhich a magneto-rheological fluid passes so as to vary an apparentviscosity of the magneto-rheological fluid to vary a damping force.JP2008-175364A discloses a magneto-rheological fluid damper where apiston assembly includes a piston core that has an outer periphery onwhich a coil is wound around and a piston ring disposed on the outerperiphery of the piston core, and when the piston assembly slides insidea cylinder, a magneto-rheological fluid passes through a flow passageformed between the piston core and the piston ring.

SUMMARY OF INVENTION

However, the magneto-rheological fluid damper described inJP2008-175364A includes a pair of plates that axially sandwiches thepiston ring, and fixes the respective plates by fastening with nuts, inorder to dispose the piston ring at a predetermined position withrespect to the piston core. Thus, having a configuration that fixes thepiston ring by sandwiching with the plates and the nuts from both ends,there is a possibility that a whole length of the piston assemblybecomes long, and a stroke length of the piston assembly becomes short.

It is an object of the present invention to shorten a whole length of apiston of a magneto-rheological fluid damper.

According to one aspect of the present invention, a magneto-rheologicalfluid damper includes a cylinder configured to seal amagneto-rheological fluid, the magneto-rheological fluid having anapparent viscosity that varies due to an action of a magnetic field; apiston slidably disposed in the cylinder, the piston defining a pair offluid chambers in the cylinder; and a piston rod coupled to the pistonto extend to an outside of the cylinder. The piston includes a pistoncore mounted on an end portion of the piston rod, the piston core havingan outer periphery on which a coil is disposed; a ring body thatsurrounds the outer periphery of the piston core, the ring body forminga flow passage for the magneto-rheological fluid with the piston core; aplate formed into a ring shape to be disposed on the outer periphery ofthe piston rod, the plate having an outer edge housed in one end of thering body, the plate being bonded on the ring body by a metal layer bybrazing; and a stopper that sandwiches the plate with the piston core.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a cross-sectional view of the front side of amagneto-rheological fluid damper according to an embodiment of thepresent invention;

FIG. 2 is a left side view of a piston in FIG. 1;

FIG. 3 is a right side view of the piston in FIG. 1;

FIG. 4 is an enlarged view of a bonding portion of a plate and a ringbody in FIG. 1; and

FIG. 5 is a cross-sectional view of the front side of amagneto-rheological fluid damper according to a modification of theembodiment of the present invention.

DESCRIPTION OF EMBODIMENTS

The following describes an embodiment of the present invention withreference to the drawings.

First, the following describes an overall configuration of amagneto-rheological fluid damper (hereinafter simply referred to as a“damper”) 100 according to the embodiment of the present invention withreference to FIG. 1.

The damper 100 is a damper that can change an attenuation coefficient bythe use of magneto-rheological fluid, which varies a viscosity accordingto an action of a magnetic field. The damper 100 is, for example,interposed between a vehicle body and a wheel shaft in a vehicle such asan automobile. The damper 100 generates the damping force that reducesvibrations of the vehicle body through extension and contraction.

The damper 100 includes a cylinder 10 that internally seals themagneto-rheological fluid, a piston 20 slidably disposed in the cylinder10, and a piston rod 21 coupled to the piston 20 to extend to an outsideof the cylinder 10.

The cylinder 10 is formed into a closed-bottomed cylindrical shape. Themagneto-rheological fluid sealed in the cylinder 10 varies an apparentviscosity by the action of the magnetic field. The magneto-rheologicalfluid is liquid produced by dispersing microparticles withferromagnetism in liquid such as an oil. The viscosity of themagneto-rheological fluid varies according to a strength of the magneticfield acting on the magneto-rheological fluid. When themagneto-rheological fluid is free from the influence of the magneticfield, the magneto-rheological fluid returns to an original state.

A gas chamber (not illustrated) to seal gas is defined via a free piston(not illustrated) in the cylinder 10. The gas chamber that is providedin the cylinder 10 compensates a volume change in the cylinder 10 byadvance and retreat of the piston rod 21.

The piston 20 defines a fluid chamber 11 and a fluid chamber 12 in thecylinder 10. The piston 20 includes a ring-shaped flow passage 22capable of moving through the magneto-rheological fluid between thefluid chamber 11 and the fluid chamber 12, and a bypass flow passage 23that is a through-hole. The piston 20 can slide inside the cylinder 10where the magneto-rheological fluid passes through the flow passage 22and the bypass flow passage 23. Details of a configuration of the piston20 will be described later.

The piston rod 21 is formed coaxially with the piston 20. An one-end 21a of the piston rod 21 is fixed to the piston 20, and an other-end 21 bextends to the outside of the cylinder 10. The piston rod 21 is formedinto a cylindrical shape where the one-end 21 a and the other-end 21 bopen. Through an inner periphery 21 c of the piston rod 21, a pair ofwirings (not illustrated) that supply current to a coil 33 a, which isdescribed later, of the piston 20 is passed. A male screw 21 d screwedwith the piston 20 is formed on an outer periphery near the one-end 21 aof the piston rod 21.

The following describes a configuration of the piston 20 with referenceto FIG. 1 to FIG. 3.

The piston 20 includes a piston core 30 including a small-diameterportion 30 a, an enlarged diameter portion 30 b, and a large-diameterportion 30 c. The small-diameter portion 30 a is mounted on an endportion of the piston rod 21. The enlarged diameter portion 30 b isformed to have a large diameter compared with the small-diameter portion30 a and axially continuous with the small-diameter portion 30 a to forma stepped portion 30 d with the small-diameter portion 30 a. Thelarge-diameter portion 30 c is formed to have a large diameter comparedwith the enlarged diameter portion 30 b and axially continuous with theenlarged diameter portion 30 b. The large-diameter portion 30 c also hasthe coil 33 a that is provided on an outer periphery thereof.

The piston 20 includes a flux ring 35, a plate 40, and a fixing nut 50.The flux ring 35 is a ring body that surrounds an outer periphery of thepiston core 30 to form the flow passage 22 for the magneto-rheologicalfluid between the piston core 30. The plate 40 is formed into a ringshape to be disposed on an outer periphery of the small-diameter portion30 a, and mounted on an one-end 35 a of the flux ring 35. The fixing nut50 is a stopper mounted on the small-diameter portion 30 a to sandwichthe plate 40 with the stepped portion 30 d.

The piston core 30 includes a first core 31, a coil assembly 33, asecond core 32, and a pair of bolts 36. The first core 31 is mounted onan end portion of the piston rod 21. The coil assembly 33 has an outerperiphery on which the coil 33 a is disposed. The second core 32sandwiches the coil assembly 33 with the first core 31. The pair ofbolts 36 are fastening members that fasten the second core 32 and thecoil assembly 33 to the first core 31.

The piston core 30 includes the bypass flow passage 23 formed by axiallypassing through the piston core 30, on a position less influenced by themagnetic field generated by the coil 33 a compared with the flow passage22. The bypass flow passage 23 includes a first through-hole 23 a and asecond through-hole 23 b. The first through-hole 23 a is formed bypassing through the first core 31. The second through-hole 23 b isformed by passing through the second core 32. The first through-hole 23a and the second through-hole 23 b are formed so as to avoid a couplingportion 33 c, which is described below, of the coil assembly 33. Thebypass flow passage 23 is formed at two positions at intervals of 180°as illustrated in FIG. 3. Not limited to this, the number of the bypassflow passage 23 may be arbitrary, or the bypass flow passage 23 may beomitted.

The first core 31 includes the small-diameter portion 30 a, the enlargeddiameter portion 30 b, a large-diameter portion 31 a that forms a partof the large-diameter portion 30 c of the piston core 30, a through-hole31 b that axially passes through the center, and the first through-hole23 a that forms a part of the bypass flow passage 23.

The small-diameter portion 30 a is formed into a cylindrical shape thataxially projects from the flux ring 35. A female screw 31 c screwed withthe male screw 21 d of the piston rod 21 is formed on an inner peripheryof the small-diameter portion 30 a. The piston core 30 is fastened tothe piston rod 21 by the screwing of the male screw 21 d with the femalescrew 31 c.

The enlarged diameter portion 30 b is formed into a cylindrical shape.The enlarged diameter portion 30 b is coaxially formed continuous withthe small-diameter portion 30 a. Between the small-diameter portion 30 aand the enlarged diameter portion 30 b, the ring-shaped stepped portion30 d is formed. The plate 40 abuts on the stepped portion 30 d. Thestepped portion 30 d sandwiches the plate 40 with the fixing nut 50. Onan outer periphery of a distal end of the small-diameter portion 30 a, amale screw 31 e screwed with a female screw 50 c of the fixing nut 50 isformed in a state sandwiching the plate 40.

The large-diameter portion 31 a is formed into a cylindrical shape. Thelarge-diameter portion 31 a is coaxially formed continuous with theenlarged diameter portion 30 b. An outer periphery of the large-diameterportion 31 a faces the flow passage 22 through which themagneto-rheological fluid passes. The large-diameter portion 31 a abutson the coil assembly 33. A cylinder portion 33 b, which is describedlater, of the coil assembly 33 is inserted into and fitted to thethrough-hole 31 b of the large-diameter portion 31 a. On thelarge-diameter portion 31 a, a pair of female screws 31 d screwed withthe bolts 36 are formed.

The first through-hole 23 a axially passes through the large-diameterportion 31 a of the first core 31. The first through-hole 23 a areformed at two positions at intervals of 180° as illustrated in FIG. 3.Damping force characteristics when the piston 20 slides are setdepending on a hole diameter of the first through-hole 23 a.

The second core 32 includes a large-diameter portion 32 a, asmall-diameter portion 32 b, a through-hole 32 c, a deep counterboredportion 32 d, the second through-hole 23 b, and a plurality of toolholes 32 f. The large-diameter portion 32 a forms a part of thelarge-diameter portion 30 c of the piston core 30. The small-diameterportion 32 b is formed on one end of the large-diameter portion 32 a,having a small diameter compared with the large-diameter portion 32 a.The bolt 36 passes through the through-hole 32 c. A head of the bolt 36is engaged with the deep counterbored portion 32 d. The secondthrough-hole 23 b forms a part of the bypass flow passage 23. A tool(not illustrated) for rotating the piston 20 is engaged with theplurality of tool holes 32 f.

The large-diameter portion 32 a is formed into a columnar shape. Thelarge-diameter portion 32 a is formed to have a diameter identical tothat of the large-diameter portion 31 a of the first core 31. An outerperiphery of the large-diameter portion 32 a faces the flow passage 22through which the magneto-rheological fluid passes. The large-diameterportion 32 a is formed such that an end surface 32 e that faces thefluid chamber 12 is a flat surface with an other-end 35 b of the fluxring 35.

The small-diameter portion 32 b is formed into a columnar shapecoaxially with the large-diameter portion 32 a. The small-diameterportion 32 b is formed having a diameter identical to that of an innerperiphery of a coil mold portion 33 d, which is described later, of thecoil assembly 33, and is fitted to the inner periphery of the coil moldportion 33 d.

A pair of through-holes 32 c are formed by axially passing through thesecond core 32. The through-hole 32 c is formed to have a large diametercompared with a diameter of an engagement portion of the bolt 36. Thethrough-hole 32 c is formed to be coaxial with the female screw 31 d ofthe first core 31 in a state where the piston core 30 has beenassembled.

The deep counterbored portion 32 d is formed on an end portion of thethrough-hole 32 c. The deep counterbored portion 32 d is formed to havea large diameter compared with the through-hole 32 c and the head of thebolt 36. The deep counterbored portion 32 d is formed having a depthcapable of completely housing the head of the bolt 36. When the bolt 36inserted through the through-hole 32 c is screwed with the female screw31 d of the first core 31, a bottom surface of the deep counterboredportion 32 d is pressed to the first core 31, and the second core 32 ispressed to the first core 31.

The second through-hole 23 b is formed to have a large diameter comparedwith the first through-hole 23 a. The second through-hole 23 b is formedat two positions at intervals of 180° as illustrated in FIG. 3. Thesecond through-hole 23 b is formed to be coaxial with the firstthrough-hole 23 a in the state where the piston core 30 has beenassembled. The hole diameter of the first through-hole 23 a decides thedamping force characteristics when the piston 20 slides. A hole diameterof the second through-hole 23 b has no influence on the damping forcecharacteristics when the piston 20 slides.

The tool holes 32 f are holes to which the tool is fitted when thepiston 20 is screwed with the piston rod 21. The tool holes 32 f areformed at four positions at intervals of 90° as illustrated in FIG. 3.In the embodiment, two of the four tool holes 32 f are formed on endportions of the second through-holes 23 b. Thus, the tool holes 32 f arecommonly used as the second through-holes 23 b.

The coil assembly 33 is formed by molding a resin with the coil 33 ainserted. The coil assembly 33 includes the cylinder portion 33 b fittedto the through-hole 31 b of the first core 31, the coupling portion 33 csandwiched between the first core 31 and the second core 32, and theannular-shaped coil mold portion 33 d that internally includes the coil33 a.

The coil 33 a forms the magnetic field by a current supplied from theoutside. A strength of this magnetic field strengthens as the currentsupplied to the coil 33 a increases. When the current is supplied to thecoil 33 a and the magnetic field is formed, the apparent viscosity ofthe magneto-rheological fluid flowing through the flow passage 22varies. The viscosity of the magneto-rheological fluid increases as themagnetic field by the coil 33 a strengthens.

In the cylinder portion 33 b, a distal end portion 33 e is fitted to theinner periphery of the piston rod 21. From a distal end of the cylinderportion 33 b, a pair of wirings for supplying the current to the coil 33a are extracted. Between the distal end portion 33 e of the cylinderportion 33 b and the one-end 21 a of the piston rod 21, an O-ring 34 asa sealing member is disposed.

The O-ring 34 is axially compressed by the large-diameter portion 31 aof the first core 31 and the piston rod 21 and is radially compressed bythe distal end portion 33 e of the coil assembly 33 and the piston rod21. This prevents an outflow and a leakage of the magneto-rheologicalfluid invaded between an outer periphery of the piston rod 21 and thefirst core 31 and between the first core 31 and the coil assembly 33 tothe inner periphery of the piston rod 21.

The coupling portion 33 c is radially disposed to extend in a straightline from a base portion of the cylinder portion 33 b to the coil moldportion 33 d, thus coupling the cylinder portion 33 b to the coil moldportion 33 d. Inside the coupling portion 33 c and the cylinder portion33 b, the pair of wirings that supply the current to the coil 33 a passthrough.

The coil mold portion 33 d is disposed into a ring shape upright on anouter circumference of the coupling portion 33 c. The coil mold portion33 d is formed to project from an end portion at an opposite side of thecylinder portion 33 b in the coil assembly 33. The coil mold portion 33d is formed to have a diameter identical to that of the large-diameterportion 31 a of the first core 31. An outer periphery of the coil moldportion 33 d forms a part of the large-diameter portion 30 c of thepiston core 30. The coil mold portion 33 d internally includes the coil33 a.

Thus, the piston core 30 is formed by being divided into the threemembers, the first core 31, the second core 32, and the coil assembly33. Accordingly, it is only necessary to only form the coil assembly 33that includes the coil 33 a by molding, and to sandwich the coilassembly 33 between the first core 31 and the second core 32. It is easyto form the piston core 30 compared with the case where the piston core30 alone is formed and molding work is performed.

In the piston core 30, while the first core 31 is fixed to the pistonrod 21, the coil assembly 33 and the second core 32 are only axiallyfitted. Therefore, in the piston 20, the second core 32 and the coilassembly 33 are fixed as pressing to the first core 31 by fastening thepair of bolts 36.

The bolt 36 is inserted into the through-hole 32 c of the second core 32to be screwed with the female screw 31 d of the first core 31. The bolt36, by its fastening power, presses the bottom surface of the deepcounterbored portion 32 d to the first core 31. This sandwiches the coilassembly 33 between the second core 32 and the first core 31. Thus, thepiston core 30 is integrated. The through-hole 32 c and the female screw31 d are formed at positions where the bolt 36 does not interfere withthe coupling portion 33 c by avoiding the coupling portion 33 c of thecoil assembly 33.

Thus, the second core 32 and the coil assembly 33 are pressed to befixed to the first core 31 by only fastening the bolt 36. This allowseasy assembly of the piston core 30.

The flux ring 35 is formed into an approximately cylindrical shape. Anouter peripheral surface 35 c of the flux ring 35 has an outer diameterformed to be approximately identical to an inner diameter of thecylinder 10. An inner peripheral surface 35 d of the flux ring 35 has aninner diameter formed to be larger than the outer diameter of the pistoncore 30. Between the flux ring 35 and the piston core 30, the flowpassage 22 is formed.

The flux ring 35 further includes an annular recess 35 e formed asaxially hollowing in a depressed shape from the one-end 35 a, and asmall-diameter portion 35 h disposed on a side of the one-end 35 a andformed to have a small outer diameter compared with the outer peripheralsurface 35 c. An axial length of the small-diameter portion 35 h is setequal to or more than an axial depth of the annular recess 35 e.

The plate 40 is a plate member formed into an annular shape. An outerperipheral surface 40 b as an outer edge of the plate 40 is pressed intothe annular recess 35 e, thus the plate 40 is housed in the annularrecess 35 e. A structure of a bonding portion of the plate 40 and theflux ring 35 will be described later in detail with reference to FIG. 4.It should be noted that the plate 40 may be housed such that the outerperipheral surface 40 b is screwed with the annular recess 35 e or isengaged with the annular recess 35 e with a backlash.

As illustrated in FIG. 2, the plate 40 includes a plurality of flowpassages 22 a, which are through-holes communicating with the flowpassage 22. The flow passages 22 a are formed into an arc shape and aredisposed at angular intervals. In the embodiment, the flow passages 22 aare formed at four positions at intervals of 90°. The flow passages 22 aare not limited to be the arc shape but may be, for example, a pluralityof circular through-holes.

Between the plate 40 and the large-diameter portion 30 c of the pistoncore 30, a bypass branch passage 25 that leads the magneto-rheologicalfluid flown from the flow passage 22 a to the bypass flow passage 23 isformed. The bypass branch passage 25 is a ring-shaped void formed on anouter periphery of the enlarged diameter portion 30 b.

The magneto-rheological fluid flown from the flow passages 22 a into thepiston core 30 flows through the flow passage 22 and the bypass flowpassage 23 via the bypass branch passage 25. Accordingly, it is notnecessary to match relative positions in a circumferential direction ofthe flow passage 22 a and the bypass flow passage 23, thus facilitatingthe assembly of the piston 20.

A through-hole 40 a to which the small-diameter portion 30 a of thefirst core 31 fits is formed at an inner periphery of the plate 40.Fitting the small-diameter portion 30 a to the through-hole 40 a securescoaxiality of the plate 40 with the first core 31.

Then, a fastening power by the fixing nut 50 to the small-diameterportion 30 a of the piston core 30 presses the plate 40 to the steppedportion 30 d to be sandwiched. This specifies an axial position of theflux ring 35 fixed to the plate 40 with respect to the piston core 30.

The fixing nut 50 is formed into an approximately cylindrical shape andis mounted to the outer periphery of the small-diameter portion 30 a ofthe piston core 30. A distal end portion 50 a of the fixing nut 50 abutson the plate 40. The female screw 50 c screwed with the male screw 31 eof the first core 31 is formed on an inner periphery of a base endportion 50 b of the fixing nut 50. This screws the fixing nut 50 withthe small-diameter portion 30 a. On an outer peripheral surface of thefixing nut 50, an engaging surface (not illustrated) with which a toolfor fastening is engaged is formed. The engaging surface has at leasttwo parallel planar surfaces. A cross-sectional outer diameter of thefixing nut 50 is, for example, a regular hexagon.

Thus, the flux ring 35 is coupled to the piston core 30 by the plate 40disposed on the one-end 35 a side of the flux ring 35 such that acentral axis of the flux ring 35 corresponds to a central axis of thepiston core 30. Furthermore, the axial position of the flux ring 35 withrespect to the piston core 30 is specified by the plate 40. Thiseliminates a need for disposing a member that couples the flux ring 35to the piston core 30 to specify the axial position of the flux ring 35on the other-end 35 b side of the flux ring 35. Accordingly, the wholelength of the piston 20 of the damper 100 can be shortened.

Since the member that couples the flux ring 35 to the piston core 30 isnot disposed on the other-end 35 b side of the flux ring 35, the flowpassage 22 is open continuously in a ring shape on the other-end 35 bside, as illustrated in FIG. 3. This reduces a flow resistance of theflow passage 22 so as to reduce a resistance provided on themagneto-rheological fluid passing through the flow passage 22.

The following describes the bonding portion of the plate 40 and the fluxring 35 in detail with reference to FIG. 4. It should be noted that, inFIG. 4, for ease of understanding, a space between the annular recess 35e of the flux ring 35 and the plate 40 is largely illustrated.

As illustrated in FIG. 4, the annular recess 35 e of the flux ring 35includes an inner peripheral surface 35 f formed to have an innerdiameter larger than that of the inner peripheral surface 35 d, and astepped portion 35 g as a bottom surface of the annular recess 35 e thatcouples the inner peripheral surface 35 f to the inner peripheralsurface 35 d.

In the plate 40 housed in the annular recess 35 e, the outer peripheralsurface 40 b is pressed into the inner peripheral surface 35 f, and aone-end surface 40 c abuts on the stepped portion 35 g. Thus, the axialposition of the flux ring 35 with respect to the piston core 30 isspecified by abutting the stepped portion 35 g of the annular recess 35e on the one-end surface 40 c of the plate 40.

As illustrated in FIG. 4, the plate 40 further includes a chamferedportion 40 e formed at a corner portion between the outer peripheralsurface 40 b and an other-end surface 40 d. In a space between thechamfered portion 40 e and the inner peripheral surface 35 f, beforebrazing, a metal used for brazing is placed.

The melted metal in brazing flows into between the outer peripheralsurface 40 b and the inner peripheral surface 35 f and between theone-end surface 40 c and the stepped portion 35 g by capillarity, andcoagulates after cooling. This forms a metal layer 60 between the outerperipheral surface 40 b and the inner peripheral surface 35 f andbetween the one-end surface 40 c and the stepped portion 35 g. In viewof this, the outer peripheral surface 40 b of the plate 40 is pressedinto the inner peripheral surface 35 f of the annular recess 35 e, andfurther, the metal layer 60 is disposed. Therefore, the flux ring 35 andthe plate 40 are strongly bonded.

It should be noted that it is only necessary that the metal layer 60 isformed at least any one of between the outer peripheral surface 40 b andthe inner peripheral surface 35 f and between the one-end surface 40 cand the stepped portion 35 g. The brazing is performed such that themetal does not leak out from a region where the flux ring 35 makes asurface contact with the plate 40.

The space where the metal used for brazing is placed is not limited tothe above-described configuration, and may be formed such that achamfered portion is disposed on the flux ring 35 side, or may be formedsuch that the chamfered portions are disposed at both of the flux ring35 and the plate 40.

The metal layer 60 is made of a copper based metal. It is not limitedthis, and depending on the materials of the flux ring 35 and the plate40, other metal such as nickel or argentum may be used.

As described above, the flux ring 35 and the plate 40 are bonded bypress fitting and the metal layer 60 by brazing. Accordingly, comparedwith a case bonded by, for example, crimping or fastening, the bond isfacilitated, and a sufficient coupling strength can be obtained.

The following describes an assembly procedure of the piston 20.

Firstly, the piston core 30 is assembled. First, the second core 32 ismounted on the coil assembly 33. The mounting is performed such that thesmall-diameter portion 32 b of the second core 32 is fitted to the innerperiphery of the coil mold portion 33 d of the coil assembly 33.

Next, the first core 31 is mounted on an assembly of the coil assembly33 and the second core 32. The cylinder portion 33 b of the coilassembly 33 is inserted into the through-hole 31 b of the first core 31from the large-diameter portion 31 a side, and the pair of wirings thatsupply the current to the coil 33 a are extracted from thesmall-diameter portion 30 a side of the through-hole 31 b of the firstcore 31. Then, the pair of bolts 36 are inserted through thethrough-holes 32 c of the second core 32 to be screwed with the femalescrew 31 c of the first core 31. This fastening of the bolts 36completes the assembly of the piston core 30.

Concurrently with the assembly of the piston core 30, the flux ring 35and the plate 40 are integrally assembled. Specifically, the outerperipheral surface 40 b of the plate 40 is pressed into the annularrecess 35 e of the flux ring 35, and then, the brazing is performed.

Here, an outer diameter of the small-diameter portion 35 h disposed onthe one-end 35 a side of the flux ring 35 is set so as not to be largerthan the outer diameter of the outer peripheral surface 40 b, even ifthe one-end 35 a side of the flux ring 35 radially bulges outside by theplate 40 being pressed into the annular recess 35 e. In view of this,even if the plate 40 is pressed into the flux ring 35, the outerdiameter on the one-end 35 a side is maintained in a state smaller thanthe outer diameter of the outer peripheral surface 40 b. As a result, asliding surface of the cylinder 10 and the piston 20 can preventoccurrence of scoring or the like. In addition, since it is notnecessary to, for example, reprocess the outer diameter of the flux ring35 in accordance with the inner diameter of the cylinder 10 after theplate 40 is pressed into the flux ring 35, the production cost can bereduced.

The brazing is performed by heating an assembly of the flux ring 35 andthe plate 40 in a state where the metal for brazing is placed in thespace between the chamfered portion 40 e and the inner peripheralsurface 35 f. At this time, if the assembly of the flux ring 35 and theplate 40 is arranged so that the other-end surface 40 d of the plate 40turns up, it can be easily visually confirmed whether the metal forbrazing is placed before brazing or not. It can be easily visuallyconfirmed from above whether the metal layer 60 is formed between theouter peripheral surface 40 b and the inner peripheral surface 35 fafter brazing or not.

Next, the plate 40 integrally assembled with the flux ring 35 isattached to the piston core 30. Specifically, the plate 40 is fitted tothe outer periphery of the small-diameter portion 30 a of the first core31 of the piston core 30 to be abutted on the stepped portion 30 d ofthe first core 31. Then, the fixing nut 50 is screwed with thesmall-diameter portion 30 a. This sandwiches the plate 40 between thefixing nut 50 and the stepped portion 30 d. In the above-describedprocedure, the piston 20 is assembled.

After the piston 20 is assembled, the piston 20 is mounted on the pistonrod 21. Specifically, the piston 20 is rotated around a central axis byfitting the tool to the tool holes 32 f. At this time, the pair ofwirings that supply the current to the coil 33 a are inserted throughthe inner periphery 21 c of the piston rod 21. This screws the femalescrew 31 c of the first core 31 of the piston core 30 with the malescrew 21 d of the piston rod 21. At this time, between the distal endportion 33 e of the piston rod 21 and the one-end 21 a of the piston rod21, the O-ring 34 is preliminarily inserted.

Thus, attaching the piston 20 preliminarily assembled to the piston rod21 facilitates the assembly of the piston 20 and the piston rod 21.

It should be noted that, in this embodiment, the piston 20 is dividedinto the three members, the first core 31, the second core 32, and thecoil assembly 33. However, instead of this configuration, the piston 20may be divided into the two members by integrally forming the first core31 and the coil assembly 33, or the two members by integrally formingthe second core 32 and the coil assembly 33.

The following describes the actions of the damper 100.

When the damper 100 extends and contracts to cause the piston rod toadvance and retreat with respect to the cylinder 10, themagneto-rheological fluid flows through the flow passage 22 and thebypass flow passage 23 via the flow passage 22 a formed on the plate 40and the bypass branch passage 25. This causes the magneto-rheologicalfluid to move between the fluid chamber 11 and the fluid chamber 12,thus the piston 20 slides in the cylinder 10.

At this time, the first core 31, the second core 32, and the flux ring35, which are made of the magnetic material, of the piston core 30constitute a magnetic path that leads a magnetic flux occurring aroundthe coil 33 a. The plate 40 is made of the non-magnetic material.Therefore, the flow passage 22 between the piston core 30 and the fluxring 35 is a magnetic gap through which the magnetic flux occurringaround the coil 33 a passes. This causes a magnetic field of the coil 33a to act on the magneto-rheological fluid flowing through the flowpassage 22 at the extension and contraction of the damper 100.

The damping force generated by the damper 100 is adjusted by a currentamount to the coil 33 a being changed so as to change strength of themagnetic field that acts on the magneto-rheological fluid flowingthrough the flow passage 22. Specifically, as the current supplied tothe coil 33 a increases, the strength of the magnetic field occurringaround the coil 33 a increases. Accordingly, the viscosity of themagneto-rheological fluid flowing through the flow passage 22 increasesto increase the damping force generated by the damper 100.

On the other hand, the bypass flow passage 23 is formed of the firstthrough-hole 23 a formed on the first core 31 of the piston core 30, andthe second through-hole 23 b formed on the second core 32 and the coilassembly 33. Between the piston core 30 and the plate 40, thering-shaped bypass branch passage 25 is defined. The bypass flow passage23 has one end that communicates with the flow passage 22 a via thebypass branch passage 25, and the other end that opens to the endsurface 32 e of the piston 20.

The bypass flow passage 23 is defined by the first through-hole 23 a andthe second through-hole 23 b that axially pass through the piston core30 made of the magnetic material. The coil 33 a is incorporated in anouter peripheral portion of the piston core 30. Therefore, themagneto-rheological fluid flowing through the bypass flow passage 23 isless likely to be influenced by the magnetic field of the coil 33 a.

Disposing the bypass flow passage 23 reduces pressure variation thatoccurs when a current value of the coil 33 a is adjusted. Accordingly,occurrence of impact, noise, and the like by rapid pressure variation isprevented. In the damper 100, the inner diameter and the length of thefirst through-hole 23 a of the bypass flow passage 23 are setcorresponding to required damping force characteristics.

With the above embodiment, the following efficiencies are provided.

In the damper 100, the plate 40 pressed into the one-end 35 a of theflux ring 35 and bonded by brazing is sandwiched between the fixing nut50 and the stepped portion 30 d of the piston core 30, and thereby theflux ring 35 is axially fixed to the piston core 30. In view of this, itis not necessary to dispose a member for fixing the flux ring 35 to thepiston core 30 on the other-end 35 b side of the flux ring 35.Accordingly, the whole length of the piston 20 of the damper 100 can beshortened.

The following describes a magneto-rheological fluid damper (hereinaftersimply referred to as a “damper”) 200 according to a modification of theembodiment of the present invention, with reference to FIG. 5. It shouldbe noted that, in the modification, components that are the same asthose in the above-described embodiment are assigned the same referencenumerals, and therefore such components will not be further elaboratedhere.

The damper 200 is different from the damper 100 according to theabove-described embodiment, in that the plate 40 is fixed using a C-ring270 as a retaining ring, not the fixing nut 50.

On the outer periphery near the one-end 21 a of the piston rod 21,corresponding to a position on which the C-ring 270 is disposed, a ringgroove 21 e formed into a shape corresponding to an outer shape of theC-ring 270 is formed.

A stopper 250 is formed into an approximately cylindrical shape to befitted to the outer periphery of the small-diameter portion 30 a of thefirst core 31. In the stopper 250, a distal end portion 250 a abuts onthe plate 40. The stopper 250 includes a tapered portion 250 c formedinto a taper shape radially expanded toward an end surface, on an innerperipheral surface of a base end portion 250 b.

The tapered portion 250 c abuts on the C-ring 270. In a state where thetapered portion 250 c abuts on the C-ring 270, the stopper 250 any morecannot axially move toward the other-end 21 b of the piston rod 21.

The C-ring 270 is a ring formed having a circular cross-sectionalsurface. The C-ring 270 is formed into a C-shaped ring shape whoseperiphery partially opens. The C-ring 270 is fitted to the ring groove21 e by a force that attempts to contract to an inner periphery. TheC-ring 270 abuts on the tapered portion 250 c of the stopper 250 tospecify an axial position of the base end portion 250 b of the stopper250.

The following describes an assembly procedure of the piston 20.

First, the flux ring 35 is preliminarily integrated with the plate 40 tobe attached to the piston core 30 integrally assembled. Specifically,the plate 40 is fitted to the outer periphery of the small-diameterportion 30 a of the first core 31 of the piston core 30 to be abutted onthe stepped portion 30 d of the first core 31. In this state, the plate40 only abuts on the stepped portion 30 d, and is not axially fixed.

Next, the piston rod 21 and the stopper 250 are assembled. First, theC-ring 270 is fitted to the ring groove 21 e of the piston rod 21. Then,the stopper 250 is fitted to the one-end 21 a of the piston rod 21. TheC-ring 270 abuts on the tapered portion 250 c on the inner peripheralsurface of the base end portion 250 b, and thereby an axial position ofthe stopper 250 is specified.

Last, the piston rod 21 and the piston core 30 are assembled.Specifically, the female screw 31 c of the first core 31 of the pistoncore 30 is screwed with the male screw 21 d of the piston rod 21. Atthis time, between the distal end portion 33 e of the piston core 30 andthe one-end 21 a of the piston rod 21, the O-ring 34 is preliminarilyinserted.

Then, as the piston core 30 is rotated with respect to the piston rod21, between the stepped portion 30 d of the first core 31 of the pistoncore 30 and the distal end portion 250 a of the stopper 250, the plate40 preliminarily attached to the piston core 30 is sandwiched. Thiscompletes the assembly of the piston 20.

Thus, a fastening power of the first core 31 of the piston core 30 withrespect to the piston rod 21 presses the plate 40 to the stopper 250 tobe fixed. Accordingly, only fastening the piston core 30 to the pistonrod 21 facilitates the assembly of the piston 20. Since the respectivemembers of the piston 20 can be firmly fixed by the fastening power ofthe piston core 30, rotation of the respective members is prevented, andvibration is reduced.

With the above modification, similarly, the plate 40 pressed into theone-end 35 a of the flux ring 35 and bonded by brazing is sandwichedbetween the stopper 250 and the stepped portion 30 d of the piston core30, and thereby the flux ring 35 is axially fixed to the piston core 30.In view of this, it is not necessary to dispose a member for fixing theflux ring 35 to the piston core 30 on the other-end 35 b side of theflux ring 35. Accordingly, the whole length of the piston 20 of thedamper 200 can be shortened.

The following summarizes configurations, actions, and effects accordingto the embodiment of the present invention.

The dampers 100 and 200 include the cylinder 10, the piston 20, and thepiston rod 21. The cylinder 10 seals the magneto-rheological fluid. Themagneto-rheological fluid has the apparent viscosity that varies due tothe action of the magnetic field. The piston 20 is slidably disposed inthe cylinder 10. The piston 20 defines the pair of fluid chambers 11, 12in the cylinder 10. The piston rod 21 is coupled to the piston 20 toextend to the outside of the cylinder 10. The piston 20 includes thepiston core 30, the flux ring 35, the plate 40, and the fixing nut 50 orthe stopper 250. The piston core 30 is mounted on the end portion of thepiston rod 21. The piston core 30 has the outer periphery on which thecoil 33 a is disposed. The flux ring 35 surrounds the outer periphery ofthe piston core 30 and forms the flow passage 22 for themagneto-rheological fluid with the piston core 30. The plate 40 isformed into the ring shape to be disposed on the outer periphery of thepiston rod 21, has the outer peripheral surface 40 b housed in theone-end 35 a of the flux ring 35, and is bonded on the flux ring 35 bythe metal layer 60 by brazing. The fixing nut 50 or the stopper 250sandwiches the plate 40 with the piston core 30.

In this configuration, the plate 40 housed in the one-end 35 a of theflux ring 35 and bonded by brazing is sandwiched between the fixing nut50 or the stopper 250, and the piston core 30, and thereby the flux ring35 is axially fixed to the piston core 30. In view of this, it is notnecessary to dispose a member for fixing the flux ring 35 to the pistoncore 30 on the other-end 35 b side of the flux ring 35. Accordingly, thewhole length of the piston 20 of the damper 100 can be shortened.

The flux ring 35 includes the annular recess 35 e formed as axiallyhollowing in the depressed shape from the one-end 35 a. The outerperipheral surface 40 b of the plate 40 is housed in the annular recess35 e.

In this configuration, the outer peripheral surface 40 b of the plate 40is housed in the annular recess 35 e. In view of this, it is notnecessary to form, for example, a projecting portion on the plate 40 toattach the plate 40 to the flux ring 35. Thus, the plate 40 can beformed into a simple flat plate shape. As a result, the production costof the dampers 100 and 200 can be reduced.

The flux ring 35 includes the small-diameter portion 35 h formed to havethe small outer diameter compared with other part, on the one-end 35 aside. The axial length of the small-diameter portion 35 h is set equalto or more than the depth of the annular recess 35 e.

In this configuration, the small-diameter portion 35 h having the lengthequal to or more than the depth of the annular recess 35 e is disposedon the one-end 35 a side of the flux ring 35. In view of this, even ifthe one-end 35 a side of the flux ring 35 radially bulges outside whenthe plate 40 is housed in the annular recess 35 e by press-fitting orthe like, on the sliding surface of the cylinder 10 and the piston 20,the occurrence of the scoring or the like can be prevented. In addition,since it is not necessary to, for example, reprocess the outer diameterof the flux ring 35 after the plate 40 is housed in the flux ring 35 bypress-fitting or the like, the production cost of the dampers 100 and200 can be reduced.

The axial position of the flux ring 35 is specified by abutting thestepped portion 35 g of the annular recess 35 e on the one-end surface40 c of the plate 40.

In this configuration, the stepped portion 35 g of the annular recess 35e abuts on the one-end surface 40 c of the plate 40 sandwiched betweenthe fixing nut 50 or the stopper 250, and the piston core 30, andthereby the axial position of the flux ring 35 with respect to thepiston core 30 is specified. Thus, the plate 40 facilitates setting ofthe axial positional relationship of the piston core 30 and the fluxring 35.

The flux ring 35 is bonded on the plate 40 by the metal layer 60 formedbetween the inner peripheral surface 35 f of the annular recess 35 e andthe outer peripheral surface 40 b of the plate 40.

The metal layer 60 is made of the copper based metal flown into betweenthe plate 40 and the flux ring 35 from the one-end 35 a side of the fluxring 35 in the melted state.

In these configurations, the melted copper based metal flows intobetween the plate 40 and the flux ring 35 from the one-end 35 a side ofthe flux ring 35, and coagulates after cooling to become the metal layer60. In view of this, the flux ring 35 and the plate 40 are stronglybonded by the metal layer 60, in addition that the outer peripheralsurface 40 b of the plate 40 is attached to the inner peripheral surface35 f of the flux ring 35 by press-fitting or the like. In theseconfigurations, the melted metal axially flows into from the other-endsurface 40 d of the plate 40. In view of this, compared with a casewhere the melted metal radially flows into from the outer peripheralsurface, before and after the brazing work, it can be easily visuallyconfirmed whether the metal for brazing is placed or not and whether themetal layer 60 is formed or not.

The flow passage 22 continuously opens in the ring shape on theother-end 35 b side of the flux ring 35.

In this configuration, since a member that couples the flux ring 35 tothe piston core 30 is not disposed on the other-end 35 b side of theflux ring 35, the flow passage 22 continuously opens in the ring shapeon the other-end 35 b side. As a result, the flow resistance of the flowpassage 22 can be reduced.

The embodiments of the present invention described above are merelyillustration of some application examples of the present invention andnot of the nature to limit the technical scope of the present inventionto the specific constructions of the above embodiments.

For example, in the dampers 100 and 200, the pair of wirings that supplythe current to the coil 33 a pass through the inner periphery of thepiston rod 21. Accordingly, an earth to let the current applied to thecoil 33 a escape to the outside can be omitted. However, instead of theconfiguration, a configuration may be employed such that only one wiringfor applying the current to the coil 33 a passes through the inside ofthe piston rod 21 so as to be earthed to the outside via the piston rod21 itself.

The present application claims a priority based on Japanese PatentApplication No. 2015-176890 filed with the Japan Patent Office on Sep.8, 2015, all the contents of which are hereby incorporated by reference.

1. A magneto-rheological fluid damper comprising: a cylinder configuredto seal a magneto-rheological fluid, the magneto-rheological fluidhaving an apparent viscosity that varies due to an action of a magneticfield; a piston slidably disposed in the cylinder, the piston defining apair of fluid chambers in the cylinder; and a piston rod coupled to thepiston to extend to an outside of the cylinder, wherein the pistonincludes a piston core mounted on an end portion of the piston rod, thepiston core having an outer periphery on which a coil is disposed; aring body that surrounds the outer periphery of the piston core, thering body forming a flow passage for the magneto-rheological fluid withthe piston core; a plate formed into a ring shape to be disposed on theouter periphery of the piston rod, the plate having an outer edge housedin one end of the ring body, the plate being bonded on the ring body bya metal layer by brazing; and a stopper that sandwiches the plate withthe piston core, the ring body includes an annular recess formed into adepressed shape axially from the one end, and the plate includes anouter peripheral surface that abuts an inner peripheral surface of theannular recess, and an end surface that abuts a bottom surface of theannular recess.
 2. (canceled)
 3. The magneto-rheological fluid damperaccording to claim 1, wherein the ring body includes a small-diameterportion formed to have a small outer diameter compared with anotherpart, on a side of the one end, and the small-diameter portion has anaxial length set equal to or more than a depth of the annular recess. 4.The magneto-rheological fluid damper according to claim 1, wherein anaxial position of the ring body is specified by abutting the end surfaceof the plate on bottom surface of the annular recess.
 5. Themagneto-rheological fluid damper according to claim 1, wherein the ringbody is bonded on the plate by the metal layer formed at least one ofbetween the inner peripheral surface of the annular recess and the outerperipheral surface of the plate, and between the bottom surface of theannular recess and the end surface of the plate.
 6. Themagneto-rheological fluid damper according to claim 1, wherein the metallayer is made of a copper based metal flown into between the plate andthe ring body from the one end side of the ring body in a melted state.